Thanks for your feedback Daniel.
This article (http://www.muzines.co.uk/articles/inside-views-yamaha/3468) says it means "Light & Music". Maybe SK and PK would be other companies or other divisions inside Yamaha.
Regarding the ID you're correct it seems to indicate a format and not an instrument thus explaining why it would change depending of the firmware's version (a newer firmware would support another SYX format etc...). This ID would be hexadecimal like many other data in the MIDI world (but still written in ASCII) so it's normal to have letters.
Regarding my TX the PSU is again in a very nice condition (no buldging/leaking caps nor tarnished solder near them) and I found that the big red wired jumper outside the PSU is actually the voltage selector! Plugged it would make it a 100V one, and unplugged a 220 (or 230V) one! I guessed that looking at the 400V capacitor (why out such a large value if your PSU is only 110V) and confirming that with the page 30 of the service manual.
